The present invention relates to methods of operating and control systems for refrigeration systems and, more particularly, to methods of operating and control systems for capcity control devices, such as compressor inlet guide vanes, in centrifugal vapor compression refrigeration systems.
Generally, refrigeration systems include an evaporator or cooler, a compressor, and a condenser. Usually, a heat transfer fluid is circulated through tubing in the evaporator thereby forming a heat transfer coil in the evaporator to transfer heat from the heat transfer fluid flowing through the tubing to refrigerant in the evaporator. The heat transfer fluid chilled in the tubing in the evaporator is normally water which is circulated to a remote location to satisfy a refrigeration load. The refrigerant in the evaporator evaporates as it absorbs heat from the water flowing through the tubing in the evaporator, and the compressor operates to extract this refrigerant vapor from the evaporator, to compress this refrigerant vapor, and to discharge the compressed vapor to the condenser. In the condenser, the refrigerant vapor is condensed and delivered back to the evaporator where the refrigeration cycle begins again.
To maximize operating efficiency, it is desirable to match the amount of work done by the compressor to the work needed to satisfy the refrigeration load placed on the refrigeration system. Commonly, this is done by capacity control means which adjust the amount of refrigerant vapor flowing through the compressor. The capacity control means may be a device such as guide vanes which are positioned between the compressor and the evaporator and which moves between a fully open and a fully closed position in response to the temperature of the chilled water leaving the chilled water coil in the evaporator. When the evaporator chilled water temperature falls, indicating a reduction in refrigeration load on the refrigeration system, the guide vanes move toward their closed position, decreasing the amount of refrigerant vapor flowing through the compressor. This decreases the amount of work that must be done by the compressor thereby decreasing the amount of energy needed to operate the refrigeration system. At the same time, this has the effect of increasing the temperature of the chilled water leaving the evaporator. In contrast, when the temperature of the leaving chilled water rises, indicating an increase in load on the refrigeration system, the guide vanes move toward their fully open position. This increases the amount of vapor flowing through the compressor and the compressor does more work thereby decreasing the temperature of the chilled water leaving the evaporator and allowing the refrigeration system to respond to the increased refrigeration load. In this manner, the compressor operates to maintain the temperature of the chilled water leaving the evaporator at, or within a certain range of, a set point temperature.
When the evaporator chilled water temperature decreases during the capacity control operating sequence described above, the guide vanes must be moved toward their fully closed position fast enough to provide a refrigeration system response which will prevent the evaporator chilled water temperature from falling below the freezing point of the water flowing through the tubes in the evaporator. This is necessary because water freezing in the tubes in the evaporator may block or break the tubes thereby possibly rendering the refrigeration system inoperable. Therefore, capacity control means for refrigeration systems are conventionally operated to drive the guide vanes toward their fully closed position at the maximum possible guide vane closing speed whenever the evaporator chilled water temperature falls below the evaporator chilled water set point temperature by a predetermined amount. No capacity control action is taken by these capacity control means before the evaporator chilled water temperature falls below the evaporator chilled water set point temperature by the predetermined amount. This is not particularly desirable since it may result in overcompensating for the decrease in the evaporator chilled water temperature thereby resulting in undesirable hunting about the evaporator chilled water set point temperature. However, this disadvantage is normally tolerated to ensure that there is no chance of the evaporator chilled water temperature falling below the freezing point of the water flowing through the tubes in the evaporator.
One control system, a model CP-8142-024 electronic chiller controller available from the Barber-Colman Company having a place of business in Rockfold, Ill., adjusts a capacity control device in a refrigeration system in a somewhat different manner than the conventional way described above. In this control system, when the evaporator chilled water temperature drops below the selected evaporator chilled water set point temperature by a predetermined amount, a capacity control device is continuously adjusted by an actuator which is continuously energized by a stream of electrical pulses supplied to the actuator. The predetermined amount of deviation before the actuator is continuously energized provides a temperature deadband in which the capacity control device is not adjusted. The pulse rate of the stream of electrical pulses supplied to the actuator determines the overall rate of adjustment of the capacity control device. This pulse rate may be set at either a minimum, middle, or maximum value thereby providing a limited capability for tailoring operation of the control system to meet specific job requirements of a particular job application for the refrigeration system. However, due to the operation of, and interrelationships among, the electrical components of the control system, the extent of the deadband depends on which pulse rate setting is selected. Also, the pulse rate is an analog function of the deviation of evaporator leaving chilled water temperature from the desired set point temperature thereby rendering this control system not particularly suitable with a microcomputer system for controlling overall operation, including capacity, of a refrigeration system.